Variable supply power amplifiers use envelope tracking to vary the supply voltage based on the amplitude of the input signal, and thereby reduce power consumption. Such amplifiers are for example used to amplify a radio frequency (RF) signal prior to transmission over a transmission channel.
FIG. 1 is a graph showing a variable supply voltage VS and the amplitude AMOUT of the corresponding RF signal at the output of a power amplifier supplied by the variable supply voltage VS.
The dashed lines in FIG. 1 illustrate the ideal output signal waveform, and demonstrate the problem of clipping at the signal peaks of the output signal. This problem occurs when the amplitude of the output signal becomes close to the level of the variable supply voltage VS, and the power amplifier thus becomes saturated, distorting the output signal. This distortion is undesirable in many applications such as in wireless communications, where it increases the bandwidth occupied by the transmitted signal.
It has been proposed to provide an adaptive predistorter that adjusts, in advance, the gain and phase of portions of the input signal before it is amplified by the power amplifier.
However, a difficulty with such adaptive predistorters is that the gain to be applied depends on several variables, for example on both the level of the input signal and the level of the variable supply voltage. Existing solutions tend to be either complex to implement, or demanding on memory resources and slow to calibrate.